1. Field of the Invention
The present invention relates to a laminated-type varistor, and particularly to a laminated-type varistor having an increased maximum peak current, an increased maximum energy, and a reduced clamping voltage.
2. Related Art
As shown in FIG. 17A, a laminated-type varistor 40, which is of a chip type and can be surface-mounted, usually comprises a laminated structure 41 and a pair of external electrodes 44a and 44b disposed on a surface of the laminated structure 41. The laminated structure 41 consists of effective sintered body layers 42, each of which provides varistor characteristics, and a plurality of internal electrodes 43a and 43b, which have a higher heat conductivity than do the effective sintered body layers 42 (see Japanese Patent Application Laid-Open (kokai) No. 54-106894.)
The laminated-type varistor 40 has been widely used, for example, as a surge-absorbing element, because of its nonlinear resistance characteristics (the varistor characteristics); i.e. when a voltage higher than a predetermined threshold is applied to the effective sintered body layer 42 via the external electrodes 44a and 44b and the internal electrodes 43a and 43b, the value of resistance of the effective sintered body layer 42 decreases considerably.
However, the laminated-type varistor described above does not have a sufficient maximum peak current or a sufficient maximum energy, and has a high contact resistance between the internal and external electrodes (connection resistance).
In addition, such a conventional laminated-type varistor releases less heat at its center portion than at the vicinity of the surface, and thus has low maximum peak current and maximum energy. Accordingly, such a varistor is easily broken due to heat generated by surge absorption. Further, since the contact resistance (connection resistance) between the internal electrodes and the external electrodes is high and therefore the clamping voltage at the time of surge absorption is high, the surge-absorbing action is not sufficient.
There has been proposed a laminated-type varistor 45 as shown in FIG. 17B, which comprises an ineffective sintered body layer 46 (a sintered body layer that is not associated with the varistor characteristics) that is thicker than an effective sintered body layer exercising the varistor characteristics and is disposed in the laminated structure (see Japanese Patent Application Laid-Open (kokai) No. 8-153606.) Since no surge-absorbing current passes through the ineffective sintered body layer 46 disposed in the laminated structure, heat is not generated due to surge absorption, and the ineffective sintered body layer serves as a heat sink layer. Accordingly, the laminated-type varistor 45 is endowed with an increased maximum peak current and an increased maximum energy.
However, the laminated-type varistor 45 still has drawbacks in that its heat-releasing property has not been sufficiently improved, its maximum peak current and maximum energy have not been sufficiently increased, and the contact resistance between the internal and external electrodes has not been decreased, so that the clamping voltage at the time of surge absorption is still high.
Another possible solution is thickening of the internal electrodes, to thereby improve their function as a heat radiation path and increase the maximum peak current and maximum energy of the varistor, and increasing of a contact area of the internal and external electrodes, to thereby reduce the contact resistance and the clamping voltage at surge absorption in order to decrease the clamping voltage at the time of surge absorption.
However, thickening of the internal electrodes has drawbacks in that the laminated structure is subject to delamination, resulting in failure to secure desired reliability.